Question: Simplify to lowest terms. $\dfrac{21}{27}$
Explanation: What is the greatest common factor (GCD) of 21 and 27? $21 = 3\cdot7$ $27 = 3\cdot3\cdot3$ $\mbox{GCD}(21, 27) = 3$ $\dfrac{21}{27} = \dfrac{7 \cdot 3}{ 9\cdot 3}$ $\hphantom{\dfrac{21}{27}} = \dfrac{7}{9} \cdot \dfrac{3}{3}$ $\hphantom{\dfrac{21}{27}} = \dfrac{7}{9} \cdot 1$ $\hphantom{\dfrac{21}{27}} = \dfrac{7}{9}$